Scanning electron microscope and CD measurement calibration standard specimen

ABSTRACT

A calibration standard specimen is provided to have formed therein calibrating patterns of a lattice shape discontinuously arrayed, and particular alignment patterns respectively disposed near the calibrating patterns so that the positioning of the specimen can be made to match the calibrating patterns to the measurement points.

BACKGROUND OF THE INVENTION

The present invention relates to scanning electron microscopes and CDmeasurement calibration standard specimens, and particularly to ascanning electron microscope and CD measurement calibration standardspecimen suited to measure dimensions of fine patterns that are formedexposed on a semiconductor wafer.

Recently, the dimensional accuracy of patterns formed on a semiconductorwafer has been reduced up to less than 100 nm, and the scanning electronmicroscope has been generally used as a pattern-dimension managing tool.There are various different needs for the performance of the apparatus,and the chief ones will be improvements in resolution/staticrepeatability accuracy, and dimension-measurement calibration precision.The demanded dimension-measurement calibration precision is one nm orbelow. Under these circumstances, an example of dimension-measurementcalibrating members is described as a dimension-measurement calibrationstandard specimen in JP-A-7-071947. In addition, another example of thedimension-measurement calibration standard specimen is disclosed inJP-A-8-031363. Moreover, an example of the critical dimension (CD)measurement calibration second-standard specimen is given inJP-A-2003-279321.

SUMMARY OF THE INVENTION

However, the dimension-measurement calibration standard specimens forthe scanning electron microscope disclosed in JP-A-7-071947 andJP-A-8-031363 have the problem that even if the dimension-measurementcalibrating patterns can be automatically addressed, it is difficult forthe pattern of interest to be again called out at hand because thepatterns are a continuous lattice pattern. In other words, the dimensionmeasurement calibration at any position cannot be made on a calibratingpattern at the same corresponding position. Although a searching guideis provided as in JP-A-2003-279321 so that the line pattern pitch can beeasily searched for, it is located above the line pattern as a functionto easily confirm the traceability chain, or traceability chainconfirming alphanumeric characters are simply added so that they can bevisually confirmed with a magnification for optical microscope. Inaddition, the function to automatically specify the measurement point isnot provided.

An object of the invention is to provide a scanning electron microscopeand dimension-measurement calibration standard specimen with the aboveproblems solved, capable of positioning the specimen to match themeasurement point so that the corresponding calibrating pattern can becalled out that was difficult in the prior art.

In order to achieve the above object, the present invention proposes ascanning electron microscope that measures dimensions of fine patternsformed exposed on a semiconductor wafer, wherein a calibration standardspecimen is used that has a plurality of dimension-measurementcalibrating patterns discontinuously arrayed, and alignment patternsformed for use in positioning the specimen to match the measurementpoint. The plurality of calibrating patterns are formed of a pluralityof line-and-space patterns discontinuously arrayed in a first directionand a plurality of lined-and-space patterns discontinuously arrayed in asecond direction perpendicular to the first direction. The alignmentpatterns are formed of combinations of patterns of at least four kindsdifferent in their shapes and arrayed such that the alignment patternsof the same shape are not adjacent to each other.

According to the invention, since the scanning electron microscope has adimension-measurement calibration standard specimen that has formedtherein a plurality of calibrating patterns discontinuously arrayed andalignment patterns provided for positioning the specimen to match themeasurement point, the specimen can be positioned to match themeasurement point at which the corresponding calibrating pattern can becalled out, and thus the dimension-measurement calibration at the sameposition can be performed. Therefore, the long-term stability of thesingle apparatus can be quantitatively evaluated. In addition, themachine matching between multiple apparatuses can be quantitativelyevaluated by using the same calibration standard specimen and the samecalibrating pattern.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram useful for explaining the basic construction of thescanning electron microscope.

FIGS. 2A through 2C are diagrams useful for explaining the constructionof the calibration standard specimen.

FIG. 3 is a diagram showing the construction of alignment patterns.

FIG. 4 is a diagram showing a method for arraying the alignmentpatterns.

FIG. 5 is a flowchart of the operation for dimension-measurementcalibration.

FIG. 6 is a diagram showing a table that holds information about thedimension-measurement calibration standard specimen.

FIG. 7 is a diagram showing an example of the input screen.

DESCRIPTION OF THE EMBODIMENT

An embodiment of the invention will be described with reference to theaccompanying drawings.

FIG. 1 is a diagram useful for explaining the basic construction of ascanning electron microscope according to the invention. A primaryelectron beam 4 emitted from an electron gun 1 is controlled/acceleratedby an anode 2, and focused/irradiated by a condenser lens 3 andobjective lens 6 onto a sample such as a semiconductor wafer or the likethat has patterns formed to be measured and that is placed on a samplestage 9. A main controller 15 controls these elements to make the aboveoperations. In the path of the primary electron beam 4, a deflector 5 isalso provided and supplied with a deflecting current from a deflectioncontroller 19. The deflecting current is changed according to themagnification that is arbitrarily set by a mouse 20 or keyboard 21connected to a computer 16. Thus, the primary electron beam 4 isdeflected by the deflector 5 to scan the surface of the sample in atwo-dimensional manner. As to the movement of the sample to themeasurement point, a stage controller 10 controls the sample stage 9 sothat the sample can be moved to an arbitrary position for themeasurement point. In addition, a template image is previouslyregistered in the computer 16 to make template matching for thepositioning of the sample to the measurement point. Therefore, anarbitrary measurement point present on the sample can be caused to comeup at hand. The secondary electrons, 7 caused by the irradiation of theprimary electron beam 4 onto the sample are detected by a secondaryelectron detector 11, amplified by an amplifier 12 and stored in animage memory 13. Then, a measurement processor 14 measures thedimensions of the pattern according to the stored image. In addition,the image signal generated at this time is displayed on a display device17.

When the scanning electron microscope is calibrated for the measurementof pattern's dimensions, a dimension-measurement calibration standardspecimen 8 is placed and measured on the sample stage 9 in place of thenormal sample. At this time, the template image registered in thecomputer 16 is used to make template matching for the positioning of thespecimen to the measurement point. Therefore, a calibrating pattern 18formed on the specimen 8 can be moved to an arbitrary measurement point.The secondary electrons 7 generated by the irradiation of electron beamonto the calibration standard specimen 8 is detected by the secondaryelectron detector 11, amplified by the amplifier 12 and stored in theimage memory 13. The measurement processor 14 measures the dimensions ofthe pattern by using the stored image. The image signal generated atthis time is displayed on the display device 17. The scanning electronmicroscope is calibrated for the measurement of dimensions bycontrolling the deflecting current and the magnification coefficient inthe measurement processor 14. A memory 34 holds information of thecalibration standard specimen 8.

The construction of the calibration standard specimen 8 will bedescribed with reference to FIGS. 2A through 2C. The calibrationstandard specimen 8 takes a form of semiconductor wafer. As shown inFIG. 2A, it has a longitudinal pattern region 23 and a lateral patternregion 24 provided at around the center. The longitudinal pattern region23 has a plurality of calibrating pattern sections 25 arrayed in atwo-dimensional manner as shown in a magnified view of FIG. 2B. Eachcalibrating pattern section 25 includes a longitudinal line-and-spacepattern 26 formed of stripes of several microns μm as shown in FIG. 2C.Therefore, the calibration standard specimen 8 has a plurality ofline-and-space patterns 26 formed discontinuously. The stripes ofline-and-space pattern 26 are formed at a pitch of about 100 nm. Thelateral pattern region 24 similarly has a plurality of calibratingpattern sections arrayed in a two-dimensional manner. Each calibratingpattern section has a lateral line-and-space pattern of stripes ofseveral microns μm that are formed at a pitch of about 100 nm.

The calibration standard specimen 8 also has global alignment purposemarks 22 formed for the translational and rotational alignment (globalalignment). An alignment pattern 27 is also provided near theline-and-space pattern 26.

According to the construction of the specimen, since the calibratingpatterns are arrayed discontinuously, the measurement position can beclearly grasped, and thus the measurement at the same position can becalibrated. The calibration standard specimen 8 may be produced as achip attached onto a specimen stub after a semiconductor wafer is dicedinto chips.

The alignment pattern will be described next. The calibrating pattern 18of the calibration standard specimen 8 placed on the sample stage 9 ispositioned to appear as the stage controller 10 controls the samplestage 9 to move. Since the positioning precision of the sample stage 9is generally a few microns μm, the final positioning of the pattern tothe measurement point is performed as follows. An image portion(template image) including a characteristic pattern as a guide ispreviously registered in the computer 16, and it is used to maketemplate matching by which the pattern position is detected, so that thepattern can be precisely positioned to the measurement point accordingto the distance from the pattern position. The line-and-space patterns26 alone could not be distinguished from their adjacent ones because thepatterns of the same shape would be present in a plural form and thuslook similar. According to the invention, the alignment pattern isprovided as the template image near the line-and-space pattern 26 on thecalibration standard specimen 8.

FIG. 3 is a diagram showing an example of the shapes of the alignmentpattern. The alignment pattern takes five different kinds of shapes:inverted T-shape 27, fallen left T-shape 28, upright T-shape 29, fallenright T-shape 30 and cross-shape 31. The image produced from thescanning electron microscope is chiefly information of edges due to theedge effect that the secondary electrons are much emitted from the edgesof patterns. Since the template matching is made in a two-dimensionalmanner, the registered template image is required to includecharacteristic information in both longitudinal and lateral directions.A numeric character formed near the measurement point is generallyregistered as a template image. For example, numerals 8 and 6 are easyto be erroneously identified because the edge characteristics aresimilar. Since the alignment patterns according to the invention haveinformation of T-shapes respectively rotated 90° in turn, theinformation of the secondary electrons emitted from the edges hastwo-dimensional characteristics, thus enabling the template matching tobe made with high precision. In addition, since the alignment patternsare respectively arranged near the featureless line-and-space patterns26, any one of the patterns 26 can be distinguished from the neighboringpatterns.

An example of the method for arranging the above alignment patterns offive kinds will be mentioned with reference to FIG. 4. The alignmentpatterns of five kinds shown in FIG. 3 are arrayed in combinations toshift, for example, left by two patters for each row of pattern sectionsas illustrated in FIG. 4. By the arrangement of the alignment patternsin such combinations, it is possible to bring about the condition thatthere are no alignment patterns of the same kind between the adjacentling-and-space patterns 26. Therefore, when the template matching isperformed under a magnification of several tens of thousands, the kindof the alignment pattern at the center within a field-of-view area 32 isdifferent from any one of the surrounding alignment patterns within thesame area 32. Thus, the wrong measurement of the adjacent line-and-spacepatterns 26 is never caused at the time of template matching. Even ifthis arrangement, in which the five different alignment patterns arearrayed in combinations in the X- and Y-directions as in FIG. 4, isrepeated endlessly, any one of the line-and-space patterns 26 always hasa alignment pattern of a kind different from those of its neighboringline-and-space patterns. Thus, according to the above arranging method,even if an endless number of line-and-space patterns 26 aretwo-dimensionally arrayed in combination with the alignment patterns ofsuch a small number of kinds as five kinds, any one of theline-and-space patterns can be distinguished from its neighboring ones.In addition, a symbol 33 for visual confirmation is formed near each ofthe line-and-space patterns 26.

The above embodiment has been described about an example of an arrayusing combinations of five different alignment patterns 27˜31. Thus, ifthe alignment patterns of more than four kinds are arranged in the samecombinations as described above, any one of the line-and-space patterns26 can be distinguished from its neighboring ones. This is because, whenthe template matching is performed under a magnification of several tensof thousands, the kind of the alignment pattern at the center within thefield-of-view area 32 is different from any one of the surroundingalignment patterns within the same area 32.

The produced calibration standard specimen is previously once examined.That is, a scanning electron microscope calibrated for dimensionmeasurement is used to previously measure all the longitudinal andlateral line-and-space patterns 26 and estimate the population varianceof the measured dimensions (pitches) of the longitudinal line-and-spacepatterns and that of the lateral line-and-space patterns. The estimatedpopulation variances are stored in the memory 34 of the scanningelectron microscope. If there are multiple calibration standardspecimens, the population variances of each specimen are stored togetherwith its ID in the table shown in FIG. 6. The used patterns will bedescribed later.

The operation for calibrating the dimension measurement in thisinvention will be explained next. FIG. 5 is a flowchart of an example ofthe operation for the calibration.

The precision of dimension-measurement calibration using the singlecalibration pattern 18 at a place is generally equal to the populationvariance of the calibration pattern 18. However, the presently-demandedprecision of dimension-measurement calibration is one nm or below, whichis smaller than the population variance of the calibration pattern 18.The precision P of dimension-measurement calibration can be expressed asP=σ/√{square root over (i)}where σ is the population variance of the calibration standard specimen8, and i is the number of measurement points.

Therefore, the precision of the dimension-measurement calibration can beraised by increasing the number i of measurement points, or by measuringmultiple calibration patterns 18 at a plurality of places andcalibrating with the average of the obtained values. If the populationvariance σ of specimen 8 and the calibration precision P are three nmand one nm, respectively, the necessary number of measurement points isnine.

The scanning electron microscope according to the invention has meansthat automatically makes initial setting of necessary number i ofmeasurement points by entering the population variance σ of specimen 8and the necessary calibration precision P through an input device suchas the mouse 20 before the specimen 8 is loaded.

In this embodiment, the operator first enters the ID of calibrationstandard specimen and the necessary precision of calibration through thekeyboard 21 while viewing the input screen shown in FIG. 7. The scanningelectron microscope searches the table of FIG. 6 stored in the memory 34for the population variances of the pitches of the longitudinal andlateral line-and-space patterns according to the ID of calibrationspecimen, and computes the necessary number of measurement points, thusmaking the initial setting (S101). In addition, in this embodiment it isassumed that a large number of arrayed line-and-space patterns shown inFIG. 4 are measured in a previously determined order. The last usedline-and-space patterns are written in the used pattern columns of thetable shown in FIG. 6. The population variances of the pitches of thelongitudinal and lateral line-and-space patterns of the standardspecimen may be directly entered into the input screen.

Then, the user operates an input device such as mouse 20 to load thecalibration standard specimen 8 into the scanning electron microscope(S102), and makes global alignment (S103). The global alignment is, forexample, to detect the known multiple global alignment purpose marks 22on the specimen 8 by an optical microscope and make translational androtational alignment. This alignment is performed for matching thecoordinate system of the specimen 8 on the sample stage 9 and the stagecoordinate system of the scanning electron microscope.

Subsequently, the first calibration point is brought about to appear(S104), where the alignment patterns are detected. It is assumed thatthe first calibration point is the pattern next to the position at whichthe used pattern is registered on the table shown in FIG. 6. Thedetection of alignment patterns (S105) is performed to follow thecalculation of the positional relation between the alignment pattern andthe measurement point of calibration pattern 18 and then the progress tothe measurement point. This detection is made by the template matchingusing the template image registered in the computer 16. The alignmentpattern used at this time is any one of the patterns 27˜31. After themovement to the measurement point, the calibration pattern 18 ismeasured (S106), and the measured value Ln is stored in the computer 16.

After the step of measurement (S106), judgment is made of whether allthe patterns in predetermined places have been measured (S107). If theabove condition is not met, the next calibration point is brought aboutto appear (S109). This operation is repeated until all the measurementpoints are completely processed. If the above condition is satisfied,the address of the finally measured longitudinal and lateralline-and-space patterns is written in the table of FIG. 6. Then, thespecimen 8 is unloaded (S111), while the computer 16 calculates theaverage value L=(L1+L2+L3+ . . . +Ln)/n from the measured values (L1,L2, L3 . . . Ln) (S108), and estimates the calibration value (S110).This calibration value is used to control the deflecting current and themagnification coefficient of the measurement processor 14, thuscalibrating the microscope. If multiple different microscopes are to becalibrated, the same calibration standard specimen 8 is used tocalibrate each apparatus so that the machine matching can bequantitatively evaluated.

Since the line width of patterns is 100 nm or below, the scanningelectron microscope that measures the critical dimensions of finepatterns formed exposed on the semiconductor wafer is demanded to havehigh degree of precision kept under strict calibration. According tothis invention, since calibration can be made at the same position as atthe time of actual measurement, the precision of the calibration in thescanning electron microscope as a single apparatus can be quantitativelygrasped. The machine matching between multiple different apparatuses canalso be evaluated.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A scanning electron microscope that irradiates an electron beam ontoa sample to scan it, detects secondary electrons generated from saidsample due to said irradiation of said electron beam, and measuresdimensions of a fine pattern formed on said sample according to an imageresulting from said detection of said secondary electrons, wherein saidmicroscope has a calibration standard specimen provided to have formedtherein a plurality of calibrating patterns discontinuously arrayed andalignment patterns respectively disposed for positioning said specimento match said calibrating patterns to measurement points.
 2. A scanningelectron microscope according to claim 1, wherein said plurality ofcalibrating patterns have a plurality of line-and-space patternsdiscontinuously arrayed in a first direction and a plurality ofline-and-space patterns discontinuously arrayed in a second directionperpendicular to said first direction.
 3. A scanning electron microscopeaccording to claim 1, wherein said alignment patterns are respectivelydisposed near said plurality of calibrating patterns.
 4. A scanningelectron microscope according to claim 3, wherein said alignmentpatterns are formed of patterns of at least four kinds different intheir shapes, and arrayed such that the alignment patterns of the sameshape are not adjacent to each other.
 5. A scanning electron microscopeaccording to claim 1, said microscope comprising: means for inputting apopulation variance of said dimensions of said calibrating patterns ofsaid calibration standard specimen or information to acquire saidpopulation variance, and a necessary calibration accuracy; and means forcalculating and setting the number of necessary measurement points onthe basis of said population variance of said dimensions of saidcalibrating patterns and said necessary calibration accuracy.
 6. Ascanning electron microscope according to claim 1, wherein saidcalibrating patterns are formed on a semiconductor wafer.
 7. Acalibration standard specimen for a scanning electron microscope, saidcalibration standard specimen having a plurality of calibrating patternsdiscontinuously arrayed, and alignment patterns respectively providedfor positioning said specimen to match said calibrating patterns tomeasurement points, wherein said plurality of calibrating patterns areformed of a plurality of line-and-space patterns discontinuously arrayedin a first direction, and a plurality of line-and-space patternsdiscontinuously arrayed in a second direction perpendicular to saidfirst direction.
 8. A calibration standard specimen according to claim7, wherein said alignment patterns are formed of patterns of at leastfour kinds different in their shapes and respectively arrayed incombination with and disposed near said plurality of calibratingpatterns such that said alignment patterns of the same shape are notadjacent to each other.
 9. A calibration standard specimen according toclaim 7, wherein said specimen is formed on a semiconductor wafer.